I have a car, a boat, and a plane. Each uses fuel. Is there a way to associate my "fuel expense" with the specific asset, or do I need 3 separate expense codes for fuel?

You only need one "fuel" expense account since your tax return does not call for a breakdown. However since you are buying three different types of fuel you certainly can have sub-accounts for Unleaded, Marine Gas, and Aviation Gas just as you might for Utilities> gas (natural) electric, water-sewer, trash.

In addition to this you can use Class Tracking. Each vehicle is a class and you can charge the on-road unleaded to the car, the marine gas (or diesel) to the boat, and the jet fuel to your F-85 SabreJet. Conceivably you might use the exact same engine lubricants in each